Semiconductor devices have been used within electronics increasingly each year ever since their initial invention in 1947. While the majority of the semiconductor market is made up of silicon-based technology, wide-band gap semiconductors such as silicon carbide and gallium nitride are finding a number of applications in RF, microwave, power switching electronics, light emitting devices such as LEDs and laser diodes, and UV detectors.
Currently, conductive thin films are available for the majority of the IR-visible range of the spectrum, however, similar films for the ultra-violet range are lacking. Such ultra-violet range films may be useful as a topside electrical contact for optical beam induced current imaging studies of defects or photoconductivity measurements in wide-band gap semiconductors, as a topside electrical contact for UV detectors, as a contact and current spreading layer for UV LEDs, as a gate contact for wide-band gap (SiC, GaN and AlN and their alloys: AlxGa1-xN) field effect transistor (FET) structures, and as rectifying and ohmic contacts for power switching diodes.
Currently, difficulties in growth have led to a wide array of extended and microscopic (in some cases only several nanometers in size) defects in wide-band gap materials which have thus hindered their marketability. Techniques such as optical beam induced current (OBIC) provide the unique capability of being able to image not only the presence of a defect, but its electrical activity under forward and reverse bias conditions and is not severely limited (as in the case of electron beam induced current) by the penetration depth of the carrier irradiation source. However, for defect analysis, OBIC requires a top and back side contact to a device. Therefore a method for allowing the above-band gap light to reach the underlying sample of interest (below the contacts) is also desirable. Since the metal contacts that are traditionally used in semiconductor technology are not transparent to light, they are not conducive for OBIC studies, thereby limiting the use of OBIC in semiconductor defect analysis. Metal contact patterns such as grids are not particularly useful for samples where the top semiconductor material layer has low conductivity, and, furthermore, they cover up some of the material being studied, hiding it from imaging. Minority carriers generated in excess of one diffusion length from a top metal contact will not be collected in the OBIC measurement, thus requiring a dense grid metal which obscures the features of interest.
Imaging of defects within wafers should be completed as early in the growth process as possible in order to save on wasted material and to study these defects during the subsequent growth and processing steps in an effort to minimize their future impact. Other conductive thin films used as transparent contacts for silicon technology, such as indium tin oxide or zinc oxide, are highly absorptive in the ultra-violet wavelengths and therefore do not allow sufficient excitation of electrical carriers for wide band gap studies. As the band gap of these materials is well into the ultra-violet (3.2 and 3.4 eV for 4H—SiC and GaN, respectively), a material that is relatively transparent in the ultraviolet wavelengths that will strongly adhere to the semiconductor surface and provide sufficient conductivity for OBIC or photoconductivity measurements to be completed would be beneficial. Furthermore, such a material would be required for UV transparent contacts needed for improving devices based on these materials that will become available as these technologies mature.
Wide bandgap optoelectronic devices such as emitters (LEDs, Lasers) and detectors (photodiodes, avalanche photodiodes) can benefit from conductive transparent contacts. Transparent contacts can significantly reduce the ‘shadow’ of a conventional metal contact and permit more light to exit (emitter) or enter (detector) the device. Transparent contacts can also be used in conjunction with metal contacts (especially in light emitting devices), to act as a current spreading layer thereby improving and optimizing the current uniformity and thus the light output uniformity over the device area. Conventional transparent conductive films, such as Indium Tin Oxide (ITO), are not sufficiently transparent in the UV and near UV spectrum.